Directional couplers are passive microwave components used for power division between transmission lines in a network. The directional property enables to couple power propagating in a certain direction. Directional couplers are a common building block in microwave systems.
There are numerous designs of directional couplers, such as coupled line sections (i.e. ¼ wavelength, multi-section or tapered), resistive bridges, Bethe hole couplers and many more. Implementation of such directional couplers may be on PCB (Printed Circuit Board), ceramic, metal structures etc. These designs are not amenable to broadband operation encompassing very low frequencies in view of the long wavelength, resulting in large dimensions and high loss at the high frequencies.
Another well-known structure for directional couplers is based on transformers. Transformers with magnetic core allow broadband operation over a range of 1:100-1:1000 in frequency, however it does not perform adequately in the microwave range in view of the deterioration in performance of magnetic components.
A directional bridge is a specific circuit implementing directional coupler functionality, used in extremely broadband applications, such as in Vector Network Analyzers test sets. A well-known design of a directional bridge is a combination of a resistive bridge and a balun structure composed of a coaxial transmission line surrounded by a ferrite material (usually the ferrite material takes the form of ferrite beads). Such structure is described in an article by Joel Dunsmore of Hewlett-Packard in “RF Design” journal of Nov. 1991, pp. 105-108, “Simple SMT Bridge Circuit Mimics Ultra-Broadband Coupler”. In VNA applications, such ferrite-based directional bridges are typically used in pairs, back-to-back, one taking a sample of the transmitted source signals, and one taking a sample of reflected or incoming signal. An example of such a dual directional bridge scheme is illustrated in U.S. Pat. No. 4,962,359 to Dunsmore entitled “Dual directional bridge and balun used as reflectometer test set”. In this patent, the two directional bridges share a common structure of a coaxial transmission line surrounded by ferrite material.
The main difficulty in realizing this design is connecting the coaxial transmission line segment (the balun) to the carrier (typically a PCB) of the resistive bridge circuit. The area needed for soldering to the PCB both the center conductor of the coaxial transmission line and the shield of the coaxial transmission line induces parasitic impedance to the ground, resulting in difficulty achieving good impedance matching at higher RF frequency (which influences the directional bridge performance).
Moreover, the use of coaxial transmission line soldered to a carrier PCB poses complexity in the production process.
It would therefore be desirable to provide an improved, cheap and easily producible device not hinging on coaxial transmission lines.
It would be further desirable to provide improved, and low cost integrated PCB-based directional bridge that overcomes at least some of the aforementioned problems with the prior art.